Data communications by wireless LAN such as IEEE802.11 are now widely used, for instance, in personal computers (PCs), PC peripherals such as printers, hard disk drives and broadband rooters, electronic apparatuses such as FAXs, refrigerators, standard televisions (SDTVs), high-definition televisions (HDTVs), digital cameras, digital video cameras and mobile phones, signal-transmitting means in automobiles and aircrafts, etc.
As the wireless LAN standard, IEEE802.11a uses an orthogonal frequency division multiples (OFDM) modulation system in a frequency band of 5 GHz, supporting high-speed data communications of maximum 54 Mbps. IEEE802.11b uses a direct sequence spread spectrum (DDSs) system in an industrial, scientific and medical (ISM) band of 2.4 GHz usable without a wireless license, supporting high-speed communications of 5.5 Mbps and 11 Mbps, IEEE802.11g uses the OFDM modulation system in a 2.4-GHz band like IEEE802.11b, supporting high-speed data communications of maximum 54 Mbps. Taking for example a case where a first communications system (11bg) is IEEE802.11b and IEEE802.11g, and a second communications system (11a) is IEEE802.11a, if necessary, explanation will be made below.
As a high-frequency circuit for use in a multi-band communications apparatus using such wireless LAN, WO2006/003959A discloses a high-frequency circuit capable of performing diversity receiving, which comprises two dual-band antennas capable of transmitting and receiving signals in two communications systems (IEEE802.11a, IEEE802.11b) with different communications frequency bands, a high-frequency switch having four ports for switching the connection of a transmitting circuit and a receiving circuit, a first diplexer circuit disposed between one port of the high-frequency switch and the transmitting circuit, and a second diplexer circuit disposed between the other port of the high-frequency switch and the receiving circuit.
The high-frequency circuit shown in FIG. 26 of WO2006/003959A is shown in FIG. 21. This high-frequency circuit comprises a diplexer circuit 13 between a high-frequency switch circuit 10 and a transmitting circuit, a power amplifier circuit 2 and a bandpass filter circuit 4 between the diplexer circuit 13 and a transmitting terminal 11bg-T; a lowpass filter circuit 19, a power amplifier circuit 3 and a bandpass filter circuit 5 between the diplexer circuit 13 and a transmitting terminal 11a-T; a detection circuit 8 between the high-frequency switch circuit 10 and the diplexer circuit 13, a diplexer circuit 14 between the high-frequency switch circuit 10 and a receiving circuit, a bandpass filter circuit 6 between the diplexer circuit 14 and a receiving terminal 11bg-R; a lowpass filter circuit 26 and a low-noise amplifier circuit 27 between the diplexer circuit 14 and a receiving terminal 11a-R; a notch circuit 28 between an antenna terminal Ant1 and the high-frequency switch circuit; and a notch circuit 29 between an antenna terminal. Ant2 and the high-frequency switch circuit 10. Although this high-frequency circuit comprises a low-noise amplifier circuit 27 in a receiving path (11a-R) of a 5-GHz band, a low-noise amplifier circuit of a 2.4-GHz band should be disposed separately, thereby being not suitable for miniaturization.
Because the diplexer circuit is connected to the input side of the low-noise amplifier, with a bandpass filter or a lowpass filter disposed between the diplexer circuit and the low-noise amplifier in WO2006/003959A, the receiving sensitivity is largely affected by the noise index of the low-noise amplifier and the insertion loss of the bandpass filter or the lowpass filter and the diplexer circuit. Accordingly, when a bandpass filter or a lowpass filter having insertion loss of about 2 dB is used, or when a diplexer circuit having insertion loss of about 1 dB is used on the input side of the low-noise amplifier, the receiving sensitivity is limited.
As a circuit commonly usable for wireless LAN and Bluetooth, JP2002-208874A discloses a circuit comprising a bandpass filter between an antenna and an antenna switch; a power amplifier commonly usable for wireless LAN and Bluetooth on the transmitting side of the antenna switch; a diplexer connected to the power amplifier to branch the transmission of wireless LAN and Bluetooth; a low-noise amplifier commonly usable for the receiving of wireless LAN and Bluetooth on the receiving side of the antenna switch; and a diplexer connected to the low-noise amplifier to branch the receiving of wireless LAN and the receiving of Bluetooth. As shown in FIG. 22, JP2002-208874A describes an example in which a frequency-selecting diplexer is constituted by a combination of a lowpass matching circuit and a highpass matching circuit. Though this circuit achieves the reduction of harmonics on the transmitting and receiving sides by one bandpass filter, it cannot be used in two frequency bands of a 2.4-GHz band and a 5-GHz band, and its receiving sensitivity is limited because it comprises a bandpass filter and a diplexer circuit on the input side of the low-noise amplifier.
Further, wireless LAN communications apparatuses adapted to IEEE802.11n to improve the speed and quality of communications by using pluralities of antennas according to a multi-input-multi-output (MIMO) technology have been becoming popular. However, the high-frequency circuits of WO2006/003959A and JP2002-208874A fail to satisfactorily handle IEEE802.11n.